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Role of MRI in Primary Rectal Cancer Staging and Management Gerard Smith Austin Radiology Overview • • • • Basic Epidemiology Basic Rectal Anatomy Key Anatomical Concepts The Role of MRI in Clinical Staging and its relevance to Treatment decisions Rectal Cancer Statistics Jemal A, Siegal R, Ward E, et al. Cancer statistics, 2009. CA Cancer J Clin. 2009;50:225-249 Rectal Cancer Statistics • Australia in 2005 – 13,076 new cases of colorectal carcinoma – 13% of all cancer cases – 4165 deaths – Lifetime risk of 1 in 12 – Rectal Cancer accounts for approx 1/3 of all colorectal cases TROG Cancer Research: trog.com.au Anatomy of the Rectum • Approximately 15 cm long • Divided into lower, mid & upper thirds • Lower: up to 5 cm from anal verge – Lies below the peritoneal reflection • Middle: 5 to 10 cm from anal verge – Peritoneal reflection extends over the anterior surface • Upper: 10 to 15 cm from anal verge – Peritoneum covers the anterior and lateral aspects of rectum • The mid to lower rectum is enveloped by the mesorectal fascia (MRF) Mesorectal Fascia (MRF) • Exists below the peritoneal reflection, extends to the pelvic floor and is an encircling fascia that contains: • • • • Rectum Perirectal fat Perirectal LN Perirectal vessels • Variable in its definition • Tapers distally • Posteriorly it lays anterior to the presacral fascia Total Mesorectal Excision (TME), and the Circumferential Resection Margin • Total Mesorectal Excision (TME) – Surgical standard for rectal excision – The rectum and the perirectal tissues en-bloc are excised – The plane of the dissection is along the outer aspect of the MRF – The Circumferential Resection Margin (CRM) is therefore in effect the MRF – TME surgery has reduced local recurrence rates significantly (from 38% to 8%) Total Mesorectal Excision The Role of MRI • To assist in clinical staging • The treatment for rectal cancer is surgery (TME) with stage appropriate neoadjuvant therapy • Intent of clinical staging is to identify patients suitable for – upfront surgical resection – neoadjuvant therapy followed by surgery • Need accurate preoperative assessment – Tumor (T) and Nodal (N) Stage – Depth of tumor invasion beyond the muscle wall – Relationship of the tumor to the MRF/CRM Neoadjuvant Therapy • Most commonly Chemoradiotherapy • Short and Long course Radiotherapy • Its addition in the treatment of locally advanced rectal carcinoma prior to TME has further reduced local recurrence rates from 8% to approximately 2% • CRT is more effective, better tolerated, associated with better compliance and less toxicity when given as neoadjuvant therapy compared with post operatively Anatomy of the Rectal Wall CIS T1 T2 • Mucosa: thin hypointense line T3 • Submucosa: thicker band of high signal • Muscularis propria: outer low signal intensity line Staging of Rectal Carcinoma: TNM • T Staging – T1: – T2: – T3: – T4: Confined to the submucosa Confined to the muscularis propria Beyond the muscularis propria Extension to involve • Visceral peritoneum (T4a) • Pelvic organs (T4b) such as prostate, seminal vesicles, cervix/uterus, bladder, pelvic side wall or pelvic floor. T Staging of Rectal Carcinoma T1: Confined to Submucosa T2: Confined to Muscularis Propria T Staging of Rectal Carcinoma • T3: Beyond Muscularis Propria T Staging of Rectal Carcinoma • T4a: Involvement of Peritoneum T Staging of Rectal Carcinoma • T4b: Involvement of Pelvic Organs T Staging of Rectal Carcinoma • Difficult to depict T1 from early T2 on MRI – Endorectal US plays a role • MRI may overcall T2 tumors as early T3 – Due to desmoplastic response in perirectal fat – Perirectal extension should be called when the tumor margin within the perirectal fat is nodular and irregular and not low intensity linear spicules T3 Staging of Rectal Carcinoma • Majority (80%) of rectal cancers present as T3 tumors – The degree of extension beyond the muscularis propria is important prognostically and potentially to the Rx chosen • • • • T3a <1mm T3b 1-5mm T3c >5-15mm T3d >15mm – Early stage T3 (<5mm) 85% 5yr cancer specific survival – Advanced stage T3 (>5mm) 54% 5yr cancer specific survival (Merkel et al. Int J Colorectal Dis 2001; 16: 298-304.) • Prospectively assessed 295 patients who underwent primary TME surgery and compared the extramural depth of invasion on MRI to histopathology • MRI and histopathology were equivalent to within 0.5mm • Note was achieved by using standardized imaging techniques, pre study imaging and pathology workshops, and standardized imaging and pathology interpretation criteria Radiology 2007; 243: 132-139 Nodal staging • N1 • N2 1-3 nodes 4 + nodes • Regional LN are: – perirectal, superior, middle and inferior rectal, sigmoid and inferior mesenteric, lateral sacral, sacral promontory, and internal iliac • External Iliac and retroperitoneal LN are not regional but represent metastatic disease Nodal staging • Nodal staging on MRI is difficult • Nodal size criteria of limited value • Nodal morphology improves accuracy – Irregular node contour – Variable signal intensity • Other techniques studied to improve accuracy include – USPIO-enhanced MRI (Lahaye MJ, et al. Radiology 2008) – Gadofosveset-enhanced MRI (Lambregts DM, et al. Abdominal Imaging 2012) • 42 TME specimens transversely sectioned and directly compared with MRI slices • 437 LN identified on pathology – 102 not seen on MRI because too small (<3mm) but only 2 of these contained metastases – 51 above the area imaged and 7 of these contained metastases • Size of benign and malignant LN similar Radiology 2003; 227(2): 371-377 • When an irregular border or mixed signal intensity used for diagnosis – Sensitivity 85% (95%CI 74% - 92%) – Specificity 97% (95%CI 95% - 99%) Radiology 2003; 227(2): 371-377 • Compared with using a 5mm cut-off – Sensitivity 68% – Specificity 78% • Systematic Review and Meta-analysis (2000-2011) – Lymph Node involvement – Sensitivity 77% (95%CI 69%-84%) – Specificity 71% (95%CI 59%-81%) Ann Surg Oncol 2012; 19:2212-2223 Mesorectal Fascial Involvement • MRI is accurate in predicting an at risk CRM • Tumor within 1 mm of the MRF is predictive of a positive CRM – – – – Advancing tumor margin Metastatic lymph node Malignant deposit Extramural Vascular Invasion (EMVI) • Low rectal tumors are at greater risk of MRF involvement • Identification of tumor involvement of the MRF modifies surgical approach Extramural Vascular Invasion (EMVI) • EMVI is the presence of tumor cells within blood vessels beyond the muscularis propria • Present in 30-40% of specimens • Associated with synchronous metastatic disease • EMVI independently predicts local and distant recurrence and poorer overall survival Retrospective study of 94 patients Sensitivity 62% Specificity 88% Relapse Free Survival at 3yrs 35% MRI-EVMI +ve 74% MRI-EVMI –ve British Journal of Surgery 2008; 95: 229-236 Summary • Understanding of the key anatomical concepts of the MRF, CRM and TME surgery • Understanding of the clinical staging of rectal cancer and the triage of patients to surgery alone or neoadjuvant CRT followed by surgery • The limitations of Nodal staging and the importance of morphology over size • Importance of assessing the tumor with respect to the MRF Thank you